Today, hearing instruments have dimensions which allow them to fit into the ear canal of a human being nearly invisible to the environment. Therefore, the dimensions of the components making up a hearing instrument have to decrease. This implies an enormous increase of the requirements of the traditional technology used during the last years. The fabrication is labour intensive, and thus very cost intensive. Furthermore, the traditional loudspeaker is shock sensitive and vibrations at higher sound levels may easily cause the well-known feedback problem of hearing instruments.
Micro-system technology (MST) provides an opportunity of batch processing which leads to low cost and good reproducibility. Full integration of electronic circuitry on the same substrate is possible and the advanced structuring technologies provide the opportunity of well-defined devices with at least a decade of better tolerances compared to traditional precision engineering. The number of publications on realised loudspeakers using MST is small and none of these loudspeakers fulfils the requirements for an application in a hearing instrument.
The loudspeaker system of a hearing instrument consists mainly of two volumes, the ear canal and the loudspeaker itself. The dimension of the ear canal and the loudspeaker is small compared to the wavelength in the considered frequency range, hence the acoustic pressure due to the sound pressure in the ear canal is approximated as quasi static. Thus, the loudspeaker is comparable to a pump. Many publications are available on this type of micro-system actuator, but issues like low supply voltages and low power consumption have not been addressed.
In order to produce a sound pressure of 106 dB SPL, the volume, V, of the ear canal (2 cm3) has to be changed by ΔV=0.0806 mm3, which corresponds to an effective pressure of about 4 Pa and a peak value of 5.6 Pa.
U.S. Pat. No. 5,960,093 discloses a miniature actuator suitable for operating as a loudspeaker in a hearing instrument. The actuator disclosed in U.S. Pat. No. 5,960,093 comprises a membrane, an armature, a cylindrical coil, permanent magnets and a drive pin in order for the armature to drive the membrane. The membrane is a stiff plate fixed on one side allowing only rotational movements. The membrane is connected to the armature by the drive pin opposite the fixed side. The armature itself is part of two parallel magnetic circuits and conducts the magnetic flux resulting from the driving voltage applied to a coil in the circuit.
A disadvantage of the actuator disclosed in U.S. Pat. No. 5,960,093 is the strong vibration resulting from the unbalanced position of the force acting point on one side of the membrane. This requires also larger deflection of the armature in order to reach the same change in volume as a membrane deflected in a position closer to the pivot leading to a lower efficiency.
It is another disadvantage of the actuator disclosed in U.S. Pat. No. 5,690,093 that the drive pin connecting the membrane and the actuator induces additional mechanical resonances to the system thereby influencing the overall performance of the actuator.
It is an object of the present invention to provide an actuator optimised for operating in environments typical for those of a hearing instrument e.g. low voltage supply and low power consumption.
It is a further object of the present invention to provide a miniature actuator having physical dimensions which allows it to fit into a hearing instrument.
It is a still further object of the present invention to provide a miniature actuator operating according to the change in reluctance principle whereby the active part of the actuator also forms a part of a magnetic path of the actuator.